Abstract
Drought is a complex natural phenomenon that occurs throughout the world. Analyzing and grasping the occurrence and development of drought events is of great practical significance for preventing drought disasters. In this study, the Standardized Precipitation Evapotranspiration Index (SPEI) was adopted as a drought index to quantitatively analyze the temporal evolution, spatial distribution, and gridded trend characteristics of drought in the Yellow River basin (YRB) during 1961–2015. The duration and severity of drought events were extracted based on run theory, and the best-fitted Copula models were used to combine the drought duration and severity to analyze the drought return period. The results indicated that: (1) the drought showed a non-significant upward trend in the YRB from 1961 to 2015, and drought events became more serious after the 1990s; (2) the month and season with the most serious drought was June and summer, with an average SPEI value of −0.94 and −0.70; (3) the seasons with an increasing drought trend were spring, summer, and autumn; (4) the most serious drought lasted for 16 months in the YRB, with drought severity of 12.44 and drought return period of 115.18 years; and (5) Frank-copula was found to be the best-fitted one in the YRB. The research results can reveal the evolution characteristics of drought, and provide reference and basis for drought resistance and reduction in the YRB.
Highlights
Drought is usually regarded as a complex periodic climate phenomenon, which will cause great losses and damage to agriculture, water resources, environment, and human life in a country or region [1,2,3]
The drought trend was different in each subzone, and the overall drought trend was gradually scale)
The drought trend was different in each subzone, and the overall drought trend was gradually obvious with the increase of time scales (Figure 3a–h)
Summary
Drought is usually regarded as a complex periodic climate phenomenon, which will cause great losses and damage to agriculture, water resources, environment, and human life in a country or region [1,2,3]. The commonly used drought index includes the Palmer Drought Severity Index (PDSI), the Water Deficit Index (WDI), the Standardized Precipitation Index (SPI), and the Standardized Precipitation Evapotranspiration Index (SPEI) [5,6,7]. The PDSI and WDI are proposed based on drought impact factors, which solve the problem. The SPI can reflect precipitation anomalies at multiple time scales, but it ignores the effect of evapotranspiration changes caused by temperature rise on drought. The SPEI combines the sensitivity of PDSI to temperature and the multi-time scale characteristics of SPI, and it has become a new robust index for scholars to study and analyze the characteristics of drought evolution. The results showed that SPEI was more suitable than SPI for reflecting the drought changes in Ningxia because it considered both precipitation and evapotranspiration.
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